Power conversion systems in Electric Vehicles (EVs) usually utilize a high energy battery pack to store energy for the electric traction system. This high energy battery pack is typically charged by a utility for an alternating current (AC) outlet. The energy conversion during the battery charging is performed by an AC/DC (direct current) converter. Such AC/DC converters, which are used to charge the high-energy battery, usually consist of two stages: (1) an input Power Factor Correction (PFC) with AC/DC conversion stage and (2) a DC/DC converter for battery charging stage. The power factor correction (PFC) is used to improve the quality of the input current that is drawn from the utility. The PFC also improves the charger, which is the isolated DC/DC converter that charges the high voltage (HV) battery.
Boost converters are generally used to realize the input PFC and AC/DC conversion. In high power applications, interleaving PFC boost stages can reduce the inductor area required, as well as reduce the output capacitor ripple current.
Conventionally, the PFC AC/DC input stage converts the AC input voltage to a fixed intermediate DC-bus voltage and then the DC/DC isolated converter is controlled according to a charging profile of the battery for the charging process. Therefore, the PFC AC/DC input converter operates independent of the charging profile of the battery. It just regulates the intermediate DC-bus voltage to a fixed value and shapes the input current of the converter. Thus the input voltage of the full-bridge converter (DC-bus) is fixed (with a second harmonic ripple). Since the input DC-bus voltage of the DC/DC converter is fixed, it operates with maximum duty-ratio at the maximum load and with very small duty-ratios at light loads. Given that the converter generally operates under full-load for a small period of time and under light loads or zero load (when battery is charged) for a longer period of time, the converter mostly operates with small duty-ratios. However, at small duty-ratios, to maintain zero voltage switching (ZVS), the amount of reactive current should be increased, but that in turn leads to higher conduction losses.
Another major drawback of the conventional AC/DC converters is the control method and system. In the conventional control technique, there is an external voltage loop to regulate the DC-bus voltage and an internal current loop to shape the input current of the converter. The voltage loop has a very low bandwidth so as not to affect the input power factor through modulation as a result of the second harmonic ripple present in the DC-bus capacitor. Typically, the cut-off frequency of such a voltage control loop is only as low as 10 Hz in order to remove the second harmonic ripple at the DC-bus voltage. Otherwise, the second harmonic would modulate the control signal at the controller output, giving rise to a third harmonic distortion of the input current. In addition, such a low bandwidth voltage control loop gives rise to a very sluggish transient response or high overshoots and undershoots in the DC-bus voltage during load transients. This causes unwanted over designing of downstream converters, which affects their efficiency and overall cost.
A boost PFC AC/DC converter is a highly nonlinear system with a large range of operation. Thus, linear PI regulators are not able to optimally perform for the whole operating range. In addition, there is another main challenge in the voltage control loop of the converter due to the presence of second harmonic ripple at the output voltage (DC-link). The voltage loop controller should be of very low bandwidth in order to remove the low frequency second harmonic ripple. Therefore, the transient response of the converter is very poor and the system usually has marginal stability against severe load changes.
The existing methods either use very high order digital comb filtering to remove the second harmonic present at the DC-link voltage or they use coarse sampling to remove the low frequency ripple.
Precise tuning of the comb filter and synchronizing of the coarse sampler are usually challenging and offset the advantages of the existing techniques. In addition, complicated digital algorithms reduce the reliability of the converter.
From the above discussion, it can be concluded that the necessity of filtering the second harmonic ripple is the prime cause of sluggish response of a conventional control methods for a PFC AC/DC boost converter.
In light of the aforementioned shortcomings of the prior art, the present invention seeks to provide an effective solution to the problems related to the conventional control systems of boost PFC AC/DC converters.